1. Field of the Invention
The present invention relates to a liquid crystal composition having a property which can be changed by controlling an applied external field such as an electric field or thermal field in a manner that is suitable for use in various types of devices. The liquid crystal composition of the present invention is useful, e.g., as a light-scattering-type light-controlling material for use in, for example, a light valve, a light-controlling glass, a recording medium, a light shutter, etc., and in devices using the same. Furthermore, the liquid crystal composition of the present invention can be used in other various applications which take advantage of the change in property.
The present invention also relates to a liquid crystal device using a liquid crystal composition.
The present invention furthermore relates to a light-scattering-type light-controlling material using a liquid crystal composition.
The present invention furthermore relates to a recording medium and a recording element using a liquid crystal composition. The recording element can be used as, for instance, a tag or a card in which information is written in the form of characters and the like, a labeling card carrying information such as bar codes, or certain types of displays.
The present invention also relates to a temperature-sensitive light shutter using a liquid crystal composition, which autonomously controls light transmittance in correspondence with the ambient temperature. The light shutter can be applied to, for instance, the sunroofs of automobiles, general use windows of houses, windows for sun-rooms and green houses, etc., to control the quantity of incident sun light so as to correspond to the temperature inside the room. The present invention is not only applicable to those described above, but is also useful in a variety of fields.
2. Description of the Related Art
So-called twisted nematic liquid crystal elements (in the liquid crystal elements of this type, the liquid crystal molecules inside the cell are twisted in the alignment direction by applying an alignment treatment to wall portions of the cell disposed opposed to each other) and ferroelectric liquid crystal elements have been heretofore developed as light-controlling elements using liquid crystal compositions. The use of polarizer sheets is essential in these elements. Disadvantages, such as their inability of provide a bright display or their narrow viewing angle, have been pointed out.
To overcome these problems, attempts have been made to develop a field-effect light-scattering light-controlling material, and its application for light-controlling or display elements has been studied. A field-effect light-scattering-type light-controlling material generally attempts to provide a light-shielding state by aligning domains of liquid crystal in random directions to realize a light-scattering state, and to provide a transparent state by applying an electric field which aligns the domains in one direction.
A light-scattering-type light-controlling material is disclosed, for example, as a "liquid crystal light modulation material" in International Unexamined Patent Publication No. WO 87/01822, or as a polymer dispersion-type liquid crystal in "liquid crystal structure and liquid crystal optical device", Japanese Patent Publication No. 52843/1991 both references incorporated herein by reference. In the polymer-dispersion-type liquid crystal, domains are formed in a transparent polymer matrix by providing a plurality of small cells in a dispersed state and filling them with low-molecular liquid crystals [the first related art].
However, because the transparent state of low-molecular liquid crystal is maintained by applying an electric field to a polymer-dispersion-type liquid crystal of the first related art above, continuous application of an electric field is necessary to keep the liquid crystal transparent. In short, the liquid crystal of the first related art has no memory function. Concerning the durability of the liquid crystal and power consumption, accordingly, a light-scattering-type light-controlling material of the type mentioned above is not advantageous.
A low-molecular liquid crystal/high-molecular liquid crystal composite is proposed as another type of light-scattering material having a memory function. Examples of this type of light-scattering material include such disclosed in T. Kajiyama et al., Chemistry Letters, (1989) pp. 817-820 incorporated herein by reference. The composite becomes transparent on turning ON a high frequency alternating current (AC) electric field, but it becomes opaque by turning ON a low frequency AC electric field. The composite exhibits a memory function, because each of the states are stably maintained even after the electric field is removed [a second related art].
However, the composite of the second related art employs a high-molecular liquid crystal which increases the viscosity of the system. Because of the high viscosity, the composite suffers from a much longer response time as compared to that of an ordinary liquid crystal system. This is the disadvantage of the composite according to the second related art.
Conclusively, by taking the first and the second related art above into consideration, no liquid crystal composition satisfying the conflicting requirements on memory function and rapid response suitable for a light-scattering-type light-controlling material has yet been made available.
Concerning a recording medium and a recording element using a liquid crystal composition, proposed are those in which information is recorded by using a laser beam or by heating. Useful rewritable recording media include those utilizing a liquid crystal. Specifically mentioned as recording media using a liquid crystal are those using a smectic liquid crystal (i.e., a group of liquid crystals having not only an ordered alignment but also a layered structure, and having a relatively inferior electric field response), a low-molecular liquid crystal such as a mixture of nematic and cholesteric liquid crystals (i.e., nematic liquid crystal is a group of liquid crystals having an ordered alignment only and a relatively favorable electric field response; cholesteric liquid crystal is a group of liquid crystals having a screw structure in an ordered alignment and an ordinary electric field response), a side-chain-type polymer, or a composite of high-molecular weight liquid crystal/low-molecular weight liquid crystal (sometimes referred to hereinafter simply as a "high-molecular liquid crystal/low-molecular liquid crystal").
In general, however, recording media using low-molecular liquid crystals are disadvantageous in terms of recording stability, and those using high-molecular liquid crystal require that a high temperature be applied to erase the information.
In the field of light shutters using a liquid crystal composition, extensive studies have been conducted, and reports have been made on various types of liquid crystal material which controls light according to changes in the temperature. For example, the study described in A. Sasaki et al., J. Appl. Phys. vol.45 (1974) p.4356 incorporated herein by reference relates to a change in light transmittance along with temperature change of a nematic liquid crystal. In A. Sasaki et al., IEEE Trans. Electron Devices, vol.ED-22 (1975) p.805 incorporated herein by reference, disclosed is a thermo-optic effect of a mixed nematic-cholesteric liquid crystal [the third related art].
In the third related art above, no material capable of becoming transparent at lower temperatures and opaque at higher temperatures has been reported up to the present time. Considering a common case of controlling temperature, if there should be a liquid crystal material capable of turning transparent at a characteristic temperature or lower to increase the amount of transmitted light and turning opaque at a temperature higher than the characteristic temperature to decrease the amount of transmitted light, the liquid crystal material can be used as a novel light shutter for controlling temperature.
Conventionally, incident sun light has been controlled by a means varying from simple ones comprising the use of, for example, a curtain, a blind, a sun shade, etc., to other types of materials taking advantage of an electro-optic effect, such as a polymer-dispersed liquid crystal, an electrochromic material, or a grain-dispersed light-controlling material, as well as a photochromic material and the like utilizing photochemical reactions.
However, except for the photochromic materials, the aforementioned related art technologies are not capable of autonomously controlling the quantity of incident light. More specifically, the quantity of incident light must be controlled by manually applying a predetermined operation, or by turning on a means for controlling the incident light according to a signal sent from a temperature sensor or the like. It can be seen therefrom that such technology not only is laborious, but also expensive, due to the cost necessary for providing a complicated constitution such as a sensor or a control means.
A photochromic material as referred to above is capable of autonomously controlling incident light. However, it is light-sensitive and not temperature-sensitive. In general, depending on the ambient temperature, people sometimes feel that intense sun light is favorable while sometimes they prefer weak sun light. Thus, a light shutter of a temperature-sensitive type is more desirable for practical applications.